Copper-zinc alloy

ABSTRACT

A copper-zinc alloy for producing electrically conductive components, for example contacts, consisting of: 62.5-67% Cu by weight, 0.25-1.0% Sn by weight, 0.015-0.15% Si by weight, at least two silicide-forming elements from the group Mn, Fe, Ni and Al with in each case being at most 0.15% by weight wherein the sum of these elements does not exceed 0.6% by weight, at most 0.1% Pb by weight, the rest being formed by Zn and also unavoidable impurities.

BACKGROUND

The present disclosure relates to a copper-zinc alloy and a copper-zinc alloy product produced from such an alloy.

The present disclosure relates to a high-strength brass alloy. High-strength brass alloys are used for producing a wide variety of products. A typical application for the use of high-strength brass alloy products are bearing parts, engine and transmission parts, for example synchronizer rings and the like, and valves, especially for drinking water applications. Brass alloy products are also used for electrical and cooling technology applications, for example for producing connector shoes, contact terminals or the like. The good thermal conductivity of brass alloy products is utilized in cooling technology applications. Owing to the well-known good thermal conductivity of copper, these brass alloys have a high copper content and correspondingly are only low alloyed. High-strength brass alloys have a significantly poorer thermal conductivity.

If a brass alloy is to have particularly good electrically conductive properties, the Cu content selected should be correspondingly high. However, the electrical conductivity of such a product decreases with increasing zinc content. For this reason, for high-strength brass alloy products for which high electrical conductivity is paramount, the alloys used typically have a Zn content of not more than 5 to 10% by weight. In addition to the elements copper and zinc, one or more of the following elements are present in the structure of high-strength brass alloys: Al, Sn, Si, Ni, Fe and/or Pb. Each of these elements has a different effect on the properties of the high-strength brass alloy product produced from the alloy. It should be noted that a single alloy element, depending on its contribution, can be responsible for different properties with regard to the processability of the alloy and with regard to the properties of a high-strength brass alloy product produced therefrom. The same applies to the processability of the alloy. Owing to the wide variety of applications of high-strength brass alloy products, a large number of high-strength brass alloys differing in their alloy composition are also known. These differ, for example, in their strength values, their machinability, their surface workability, their thermal conductivity, their modulus of elasticity, their temperature resistance and the like. In most cases, the known high-strength brass alloys have been developed with regard to their composition for very specific purposes.

A high-strength brass alloy from which high-strength brass alloy products for electrical applications are to be produced, must not only have sufficient electrical conductivity, but in order to be able to produce the desired products, must also have good processability and workability, as well as sufficient strength values. With regard to the processability of the alloy, its production should be possible using standard processing steps, so that the costs of the high-strength brass alloy products produced therefrom are not made more expensive by costly and possibly unusual process control steps.

A high-strength brass alloy for electrical and/or cooling technology applications has become known from DE 20 2017 103 901 U1. This contains 58.5-62% by weight Cu, 0.03-0.18% by weight Pb, 0.3-1.0% by weight Fe, 0.3-1.2% by weight Mn, 0.25-0.9% by weight Ni, 0.6-1.3% by weight Al, 0.15-0.5% by weight Cr, at most 0.1% by weight Sn, at most 0.05% by weight Si, the rest being formed by Zn and also unavoidable impurities. Although this high-strength brass alloy has sufficient thermal conductivity for the intended cooling technology applications and sufficient electrical conductivity for many applications, it would be desirable if not only the electrical conductivity but also the extrudability and machinability could be improved in order to improve the producibility of electrical components such as contacts, sockets or the like. In addition, the alloy product produced from such an alloy should have good cold formability properties, such as good cold drawability properties, so that in this way the formed semi-finished product is provided with higher strength values for the end product.

A lead-free brass alloy with good machinability is known from US 2014/0234411 A1. This alloy comprises 70-83% by weight Cu, 1-5% by weight Si and the following other matrix-active elements: 0.01-2% by weight Sn, 0.01-0.3% by weight Fe and/or Co, 0.01-0.3% by weight Ni, 0.01-0.3% by weight Mn, the rest being formed by Zn and also unavoidable impurities. In addition, the alloy can contain up to 0.1% by weight P as well as the elements Ag, Al, As, Sb, Mg, Ti and Cr with, in each case, at most 0.5% by weight.

A copper-zinc alloy as a material for electronic components is known from DE 41 20 499 C1. This alloy from the prior art comprises 74-82.9% by weight Cu, 1-2% by weight Si, 0.1-0.4% by weight Fe, 0.02-0.1% by weight P, 0.1-1.0% by weight Al, the rest being formed by Zn and also unavoidable impurities.

Brass alloys, which are said to have good electrical conductivity, are produced with a high Cu content. The alloy according to DE 41 20 499 C1 is one such example. This brass alloy from the prior art has a rather high mechanical strength, a high spring bending yield strength and thus a corresponding modulus of elasticity, so that resilient connector parts can be produced from this alloy. However, despite the high Cu content, the electrical conductivity is only between 6.0-7.0 mS/m.

DESCRIPTION

Proceeding from this background, an aspect of the present disclosure is therefore to propose a high-strength brass alloy that is particularly suitable for producing electrically conductive components, for example contacts as parts of connectors, which is characterized by improved mechanical properties and an improved electrical conductivity. In addition, it should have good machinability and good cold formability properties.

According to the present disclosure, this aspect is provided by a copper-zinc alloy for producing electrically conductive components, for example contacts, consisting of:

-   -   Cu: 62.5-67% by weight,     -   Sn: 0.25-1.0% by weight,     -   Si: 0.015-0.15% by weight,     -   at least two silicide-forming elements from the group Mn, Fe, Ni         and Al with, in each case, at most 0.15% by weight, wherein the         sum of these elements does not exceed 0.06% by weight,     -   Pb: at most 0.1% by weight,     -   the rest being formed by Zn and also unavoidable impurities.

This copper-zinc alloy is characterized by its particular alloy composition. On the one hand, the determining factor is the Zn content of 31-37% by weight and the significant contribution of the element Sn to the composition of the alloy with 0.5-1.0% by weight. The main alloy elements of this alloy are the elements Cu, Zn and Sn. Owing to the relatively high Zn content and the correspondingly lower Cu content, it was surprising to find that the electrical conductivity nevertheless meets the requirements placed on a product made from this alloy and even exceeds the conductivity of high-strength brass alloys from the prior art which have been used for electrically conductive applications. Si is present in the alloy with 0.015-0.15% by weight. The Si in the alloy serves to form silicides as fine precipitates in the microstructure. The average size of the silicides is typically less than 1 μm. If the silicides exceed a certain size, this has an adverse effect on the polishability, coatability and/or solderability of the surface of the alloy product produced from the alloy. A higher Si portion cannot improve the particular properties of the alloy according to the present disclosure. Rather, this could adversely affect the desired good electrical conductivity. From the group of elements Mn, Fe, Ni and Al as silicide-forming elements, at least two elements are present in the structure of the alloy. Together with Si, these elements form finely distributed mixed silicides, which have a positive effect on the abrasion resistance of the product produced from the alloy. These silicides are finely distributed particles in the microstructure matrix. The portion of these elements in the structure of the alloy is limited to at most 0.15% by weight per element, wherein the sum of these elements does not exceed 0.6% by weight. Preferably, the elements Fe, Ni and Al are present in the structure of the alloy. Mn can be part of the alloy as a silicide former. Preferably, the elements Fe, Ni and Al are provided as silicide formers, which typically form mixed silicides. It is provided in one embodiment that the Ni and Al portions are in each case approximately equal in size, while the Fe portion is only 40-60% of the Ni and Al portions. In a preferred embodiment, the Fe portion is approximately 50% of the Ni or Al portion. This particular combination of the silicide formers Fe, Ni and Al together with the Si content of between 0.015-0.15% by weight does not have any appreciable disadvantageous effect on the desired particularly good electrical conductivity of the product produced from the alloy. Nevertheless, these give the alloy product the desired strength values.

Unexpectedly and surprisingly, it has been shown with this alloy or an alloy product produced from this alloy that it not only has a particularly fine grain (typically 10-100 μm), but that it also has very good extrusion or hot forming properties, has good work hardening properties by cold forming and has good machinability, and nevertheless has a very good electrical conductivity of more than 12 mS/m (20% IACS) for high-strength brasses of the type under discussion. This is also owing to the relatively high Sn portion with simultaneously limited portions of the silicide-forming elements.

In general, the prevailing theory was that brass alloys, which should have good machinability, must not have a copper content below 70% by weight (see, for example, US 2014/0234411 A1). In this respect, it was surprising to find that, despite the low copper content, the alloy according to the present disclosure or the product produced therefrom has very good machinability.

What is of interest for electrical applications of a high-strength brass alloy product produced from this alloy is its particularly good galvanic coatability. In some applications, such products are coated with an electrically highly conductive metal layer, i.e. a coating whose electrical conductivity clearly exceeds that of the product produced from the brass alloy. Such a metal layer is typically applied galvanically. This not only requires a certain conductivity of the high-strength brass alloy product, but above all that a galvanic application applied to it adheres permanently and evenly over the surface. This is due in particular to the uniform, fine-grained microstructure that occurs with this high-strength brass alloy. This is the case with products produced from this alloy. A coating of the brass alloy product can also serve to protect against wear. Furthermore, coatings can be used to improve certain properties of the brass alloy product on the surface, such as better solderability, for example for attaching contacts, thermal insulation for thermal protection of the high-strength brass alloy product or as an adhesive layer for a further coating.

In addition, the modulus of elasticity of a product produced from this alloy is sufficiently high. This brass alloy can therefore also be used for producing products with resilient properties, for example connector shoes as contacts. With a modulus of elasticity of more than 100 to 120 GPa, this is in the size range of the moduli of elasticity known from low-alloy copper-zinc two-substance alloys, as are typically used for electrical applications which sometimes also involve the application of spring force.

This brass alloy can be used to produce alloy products that have an electrical conductivity of more than 12 mS/m (20% IACS). This results in electrical conductivity values that are generally higher than in other high-strength brass alloys with a Zn portion of 30% by weight and more and which are sufficient for many applications. In alloy products produced from this alloy, this is combined with strength values which are otherwise only known from high-strength brass alloys specially designed for this purpose, but which then do not have the other positive properties of this alloy or a product produced therefrom.

The good solderability of the high-strength brass alloy product produced from this high-strength brass alloy is not insignificant, especially in electrical applications.

The simple chemical structure of this copper-zinc alloy should be emphasized owing to the small number of elements present in the structure of the alloy. This also means that the alloy is Cr-free. The alloy is also typically Pb-free, wherein a Pb portion of at most 0.1% by weight is permitted. It cannot always be avoided that small amounts of Pb are introduced into the alloy owing to carryover or the use of recycled material. Within the permitted range, Pb does not have a negative effect on the positive properties of this copper-zinc alloy as described above. With a maximum permitted Pb portion of 0.1% by weight, this alloy is still considered to be Pb-free. Furthermore, elements such as P, S, Be, Te and others are not used-elements which are often used in addition to Cr in other high-strength brass alloys to achieve certain strength or processing properties. This is also the reason for the surprising result that the above-described positive properties of a product produced from the alloy occur, even though the alloy is composed of only a few elements, provided that the elements are present in the alloy with the specified portions. The use of only a small number of elements in the structure of the alloy simplifies the production process. The risk of element carryover for other alloys is avoided in commercial production, since the elements present in the structure of the alloy are standard elements of every high-strength brass alloy.

The particularly good machinability of an alloy product produced from this alloy can be specified with an index of 60-70 and, in a special version, of more than 80.

The copper-zinc alloy according to the present disclosure preferably has the following composition:

-   -   Cu: 64-66% by weight,     -   Sn: 0.3-0.7% by weight,     -   Si: 0.03-0.1% by weight, with which alloy composition the         positive properties of the alloy are further improved.

According to one embodiment, the portion of Sn and Si is further restricted, as is the portion of silicide-forming elements. Such an alloy is composed as follows:

-   -   Cu: 64.5-66% by weight,     -   Sn: 0.4-0.6% by weight,     -   Si: 0.03-0.08% by weight,     -   at least two silicide-forming elements from the group Mn, Fe, Ni         and Al with, in each case, at most 0.1% by weight, wherein the         sum of these elements does not exceed 0.4% by weight,     -   Pb: at most 0.1% by weight,     -   the rest being formed by Zn and also unavoidable impurities.

The preferred Zn content is between 32 and 36% by weight.

The description below refers to an example embodiment in comparison with three comparative alloys. The alloy according to the present disclosure was produced and extruded on the basis of two samples—samples A and B—in addition to three comparative alloys. The composition of the investigated alloys is shown in the table below:

Cu Pb Sn Fe Mn Ni Al Si Cr Zn A 65 — 0.5  0.035 — 0.07 0.07 0.06 — Rest B 65.05 — 0.45 0.04 — 0.14 — 0.03 — Rest 1 60.3 0.11 — 0.5 0.8 0.5 0.9  — 0.24 Rest 2 60 0.1 0.08 0.05 0.025 0.01 0.03 0.005 0.01 Rest 3 58.3 0.1 0.08 0.1 0.008 0.01 0.01 0.005 0.02 Rest (Figures in % by weight)

In the table above, the comparative alloys are alloy 1, alloy 2 and alloy 3. In the extruded state, the alloy according to the present disclosure according to samples A and B has the following strength values:

-   -   0.2% tensile yield strength: 100 N/mm²,     -   tensile strength: approx. 300 N/mm²,     -   elongation at break: approx. 55%,     -   hardness: 70 HB 2.5/62.5

The good cold drawability and the associated work hardening, which results in increased strength values in the alloy product, can be demonstrated in the cold drawn state of the extruded bar in a first step with a 20% reduction in cross section and in a second step with a 35% reduction in cross section (see also FIGS. 1 to 5):

Strength values of the cold drawn bar with a 20% reduction in cross section:

-   -   0.2% tensile yield strength: approx. 310 N/mm²,     -   tensile strength: approx. 390 N/mm²,     -   elongation at break: approx. 25%,     -   hardness: approx. 120 HB 2.5/62.5.

Strength values of the cold drawn bar with a 35% reduction in cross section:

-   -   0.2% tensile yield strength: approx. 400 N/mm²,     -   tensile strength: approx. 450 N/mm²,     -   elongation at break: 12%,     -   hardness: 143 HB 2.5/62.5.

The microstructure of the alloy according to the present disclosure predominantly shows the α phase in the matrix at room temperature. At hot forming temperatures, there is a sufficient portion of the β phase. The grain microstructure is small at room temperature with an average grain size of 10 to 100 μm. The silicides are finely distributed as fine precipitates which form at extrusion temperatures.

The properties of alloy samples A and B according to the present disclosure at room temperature in comparison to the three comparative alloys are shown in the table below for a partially solidified state in each case, as is customary for the production of connectors:

Alloy samples Unit Alloy 1 Alloy 2 Alloy 3 A and B Extrudability Good Good Good Very good Cold drawability Good Very good Good Very good Machinability Index  80  20  25 ≥80  Electrolytic Good Very good Average Good polishing Galvanic Very good Very good Good Very good polishing Thermal [W/(m*K)] 100-110 385 approx. ≥100  conductivity 310 Electrical [mS/m]    9.1  56 ≤43  approx. 14 conductivity (20% IACS) Modulus of [GPa]  96 107 110-130 100-120 elasticity 0.2% tensile [MPa] approx. approx. approx. 410 yield strength 550 240 350 Tensile strength [MPa] approx. approx. approx. 450 650 280 420 Elongation at [%] approx. approx. approx.  25 break  15   8  8

This comparison shows that the alloy according to the present disclosure has particularly good properties with the parameters relevant for electrical applications. This is also associated with a particularly high modulus of elasticity and very good strength values. For this reason, this alloy is particularly suitable for the production of electrical contact elements which must have material-elastic properties.

Investigations on casting samples of alloy samples A and B according to the present disclosure show that the β-mixed crystal portion is quite low at 12-15%, the rest being the α-mixed crystal portion. The portion of intermetallic phases is less than 1%. The high portion of a phases during casting has a positive effect on the subsequent cold forming steps. If hot forming is desired, efforts will be made to keep the β phase portion somewhat higher.

As a result of extrusion, the β portion is reduced to below 2%. The density is 8.58 g/cm³. The electrical conductivity in the extruded state of these samples is 13.8 mS/m (23.8% IACS). These samples have a hardness of about 80 HB 2.5/62.5.

When a stress corrosion cracking test was carried out in accordance with DIN 59016 Part 1, no stress cracks occurred. This means that in the extruded state, there is no residual stress in the microstructure, at least no significant residual stress. This result is consistent with the high homogeneity of the microstructure and the small grain, which has been confirmed by micrographs. The particular microstructure of such an alloy product with its predominant α phase is responsible for the good electrical conductivity described above. In addition, owing to the homogeneous microstructure, not only are the mechanical properties in different directions the same, but so is the electrical conductivity.

The electrical conductivity can be improved by carrying out a subsequent annealing step, which is preferably carried out at between 380° C. and 500° C. for about 3 hours. Annealing is preferably carried out at temperatures between 440° C. and 470° C. for 3 hours. By means of annealing, fine precipitates are removed because they hinder the electrical conductivity. After annealing, an electrical conductivity of about 14.2 mS/m was measured for samples A and B.

Another particular advantage of the alloy according to the present disclosure is its particularly good cold formability. Semi-finished products produced therefrom can also be cold formed several times without intermediate annealing, for example elongated or bent, in order to give the component particularly high strength values as a result of the occurring work hardening.

The accompanying FIGS. 1 to 5 show diagrams from which the mechanical strength properties of the alloy according to the present disclosure are established on the basis of sample A with increasing elongation of the specimen. The elongation relative to the starting surface or starting length of the specimen is plotted on the x-axis.

FIG. 1 shows the development of the 0.2% tensile yield strength of the specimen with increasing elongation, up to a total elongation of 60%. The 0.2% tensile yield strength increases with increasing elongation of the specimen. The same reaction can also be found regarding the tensile strength (see FIG. 2). The elongation carried out as cold forming leads to an increase in the tensile strength by more than 100% if the specimen has been elongated by over 50%. An increase in the yield strength ratio can also be observed with increasing elongation of the specimen (see FIG. 3).

The elongation at break is of particular interest for the alloy. Despite elongation even in areas of over 50% and thus despite strong deformation, the elongation at break does not fall below 10% (see FIG. 4).

With the increasing elongation of the specimen, the hardness increases owing to the associated cold deformation, namely up to about 180 HB 2.5/62.5 (see FIG. 5).

These diagrams illustrate the particularly good cold formability properties of a product produced from the alloy according to the present disclosure. 

1-13. (canceled)
 14. Copper-zinc alloy for producing electrically conductive components consisting of: Cu: 62.5-67% by weight, Sn: 0.25-1.0% by weight, Si: 0.015-0.15% by weight, at least two silicide-forming elements from the group Mn, Fe, Ni and Al, each at most 0.15% by weight, wherein the sum of these elements does not exceed 0.06% 0.6% by weight, Pb: at most 0.1% by weight, remainder Zn and also unavoidable impurities.
 15. Copper-zinc alloy of claim 14, with Cu: 64-66.5% by weight, Sn: 0.3-0.7% by weight, Si: 0.03-0.1% by weight.
 16. Copper-zinc alloy of claim 15, with Cu: 64.5-66% by weight, Sn: 0.4-0.6% by weight, Si: 0.03-0.08% by weight, at least two silicide-forming elements from the group Mn, Fe, Ni and Al, each at most 0.1% by weight, wherein the sum of these elements does not exceed 0.4% by weight, remainder Zn and also unavoidable impurities.
 17. Copper-zinc alloy of claim 14, wherein the alloy contains Zn with 32-36% by weight.
 18. Copper-zinc alloy of claim 14, wherein the silicide-forming elements in the alloy contain Fe, Ni and Al, and wherein the Ni portion and the Al portion are approximately equal, and the Fe portion is 40% to 60% of the Ni portion or of the Al portion.
 19. Copper-zinc alloy of claim 18, wherein each of the Ni content and the Al content is 0.04 to 0.1% by weight, and the Fe content is 0.02 to 0.05% by weight.
 20. Copper-zinc alloy of claim 19, wherein each of the Ni content and the Al content is 0.06 to 0.08% by weight, and the Fe content is 0.03 to 0.04% by weight.
 21. Copper-zinc alloy of claim 14, wherein the alloy is Cr-free.
 22. Copper-zinc alloy product produced from a copper-zinc alloy according to claim 14, wherein the microstructure matrix at room temperature has a strongly predominant α phase.
 23. Copper-zinc alloy product of claim 22, wherein the average grain size of the microstructure is between 10 and 100 μm.
 24. Copper-zinc alloy product of claim 22, wherein its electrical conductivity is at least 12 mS/m (20% IACS).
 25. Copper-zinc alloy product of claim 22, wherein the product is cold-formed from a semi-finished product by drawing, with a reduction in cross section of about 20%, and exhibits the following strength values: 0.2% tensile yield strength: approx. 310 N/mm2, tensile strength: approx. 390 N/mm2, elongation at break: approx. 25%, hardness: approx. 120 HB 2.5/62.5.
 26. Copper-zinc alloy product of claim 22, wherein the product is cold-formed from a semi-finished product by drawing, with a reduction in cross section of about 35%, and exhibits the following strength values: 0.2% tensile yield strength: approx. 400 N/mm2, tensile strength: approx. 450 N/mm2, elongation at break: 12%, hardness: 143 HB 2.5/62.5. 